This invention relates to a self-expanding stent graft for percutaneous transluminal delivery. Such a prosthesis can be regarded as a “lining” for a short length of a bodily lumen. Such lumens are often arteries but need not be. For example, stents are often used in the ureter or in the biliary tree. A prosthesis of larger diameter could be placed in the oesophagus or the trachea.
Up until now, these prostheses have had a relatively short length, and are used for treating short lengths of bodily lumen, where disease is local. The present inventor has realised that it would be useful to have a prosthesis with a length significantly longer than that available with the current stents and grafts. Specifically, the present inventor has recognised the usefulness of a stent graft having a length of at least 200 mm, and has realised how this objective can be accomplished.
In the early days of development of percutaneous, catheter-delivered stents for bodily lumens, there were proposed stainless steel cages to be mounted on a balloon and expanded at the site of stenting by inflation of the balloon in the stent lumen, to cause plastic deformation of the struts of the stainless steel cage, to achieve one way expansion of the radius of the cage, whereupon deflation of the balloon would allow the delivery system to be removed, leaving the expanded stent cage in place. Thereafter, it was proposed to cover the stainless steel cage stenting rings, to make available corresponding stent grafts.
Another form of stent, also of stainless steel, was the self-expanding form of stent, contained within a sheath until proximal withdrawal of the sheath would allow the resilient stenting ring to expand radially by elastic deformation, again allowing the delivery system thereafter to be withdrawn proximally.
The number of materials that are biologically compatible, yet with mechanical characteristics strong enough for stenting, is severely limited. Besides stainless steel, another available material is a nickel-titanium shape memory alloy which goes under the trade mark NITINOL. Stents of this material are self-expanders, again confined by a radially-overlying sheath which is withdrawn proximally to release the stent to expand radially. Another candidate is a cobalt-chromium-nickel alloy identified by the trademark PHYNOX.
It will be appreciated that proximal withdrawal of a surrounding sheath of a self-expanding stent requires a tension force in the sheath which, other things being equal, increases as the axial length of the stent increases, since the entire length of the stent within the sheath is, at the site of stenting, imposing radially outward pressure on the sheath, thereby raising the forces of friction acting between the stent and the sheath. Similarly, for the operation of loading the self-expanding stent into a suitable catheter-based delivery system, some means has to be found for advancing the sheath over the length of the stent, or pulling the stent into the sheath, until its full length is within the sheath.
The forces of friction have, until now, imposed an upper limit on the overall dimensions of length and diameter of a self-expanding stent graft that can feasibly be installed in a surrounding sheath, and then released from the sheath at the site of stenting.
The present inventor has found a way to install and deliver self-expanding stent grafts of greater dimensions than hitherto, in particular, stent grafts of greater length than hitherto.
International Patent Application No. PCT/EP2004/004486 (PCT 486), published as WO 2004/096091, is from the present inventor and declares the priority of British Patent Application No. 0309616.1 of the present inventor. The present specification includes disclosure from the description, claims and drawings of PCT/EP2004/004486.
From consideration of PCT 486, it is immediately apparent that a feature useful for loading and delivery of extra long self-expanding stent grafts, covered stents and other implants is an element that sits within the lumen of the stent graft and is characterised by a spiral formation on its radially outward-facing cylindrical surface. Edge surfaces which define the spiral form engage with the luminal face of the graft or cover material of the implant, throughout the length of the implant. In this way, axial force can be passed between the implant and the element within its lumen, along the full length of the implant, via the spiral form edges. It is this distribution of axial forces along the full length of the implant which carries, along the full length, the friction forces operating between the abluminal surface of the implant and any surrounding sheath of a delivery system for the implant.
With the use of such a means to distribute friction forces along the full length of the implant, it is envisaged that implants of significantly more than 200 mm length could be deployed. Specifically, it is envisaged to deploy, with the spiral form element mentioned above and shown in the annex to the specification, stent grafts of around 500 mm in length. Thus, for the first time, it becomes feasible to contemplate a stent graft lining for, for example, the femoral-popliteal artery of the human leg, along a substantial or even major part of its length.
DE-A-10016920 discloses electroplating a metal mesh workpiece with a length of 5 to 200 mm prior to using the workpiece to make medical implants.
FR-A-2760351 discloses making from filament a helical pattern stent which may have a length more than 20 mm, or even more than 50 mm.
FR-A-2742042 discloses making a vascular prosthesis to replace natural bodily tissue and with a length which may be as much as 120 cm.
WO 03/049641 discloses a radially expandable stent which consists of a plurality of axially spaced radially expandable cylindrical stenting rings aligned along a common axis to define a lumen and arranged around a polymer tube. The tube provides longitudinal and flexural flexibility to facilitate transluminal advance and delivery through a tortuous bodily lumen, and the rings provide the strength to maintain patency of the lumen
EP-A-788 332 discloses a self-expanding braided metallic stent tube and a delivery system that includes a soft annulus within the stent lumen that deforms and mechanically engages with the mesh of the stent for restraining the stent from axial movement relative to the inner catheter of the delivery system, during axial movement of a sleeve surrounding the stent. The disclosure of EP-A-596 145 is similar.
EP-A-836 447 discloses a system for delivering a self-expanding stent, in which a stopper ring on an inner catheter abuts the proximal end of the stent tube during proximal withdrawal of a sheath which surrounds the stent.
The tubular envelope of a stent usually has apertures through its wall thickness to permit radial expansion. Thus, an uncovered or “bare” stent has a tube wall that is normally liquid-permeable. However, there are many occasions when a stent with a liquid-impermeable wall that is not apertured would be desirable. To meet these needs, a family of “covered” stents have been developed. Applicant has particular experience with stent tubes provided with a covering of expanded polytetrafluoroethylene (ePTFE). Typically, the stent tube is covered by luminal and abluminal covering layers of ePTFE, which are bonded to each other through the apertures in the stent tube wall.
During manufacture of stents and delivery systems, attention must be paid to sterility. Specifically, one needs procedures for loading a covered stent into a catheter delivery system that will allow sterile conditions to be maintained, or at least thereafter achieved.
Typically, to introduce a covered self-expanding stent into a catheter delivery system, a tool needs to be provided that compresses the covered stent radially inwardly, down to a diameter which is smaller than the available diameter of the lumen of the delivery system that is to receive the compressed covered stent. Clearly, any structure within the lumen of the stent that resists further inward compression is better avoided, when the objective is to compress the stent radially inwardly as much as the system will tolerate, so as to keep the outside diameter of the delivery system at its distal tip as small as possible.
However, the stent has to be maintained at the stenting site during proximal withdrawal of the surrounding sheath, for progressive release of the stent at the stenting site. If there is no structure within the lumen of the stent, then the entire stress imposed on the stent, to prevent it moving proximally with the proximally withdrawing surrounding sheath, has to be carried on the proximal end annulus of the compressed stent. Often this is not really a problem, especially when the stent is short and not particularly highly compressed radially inwardly, and especially when friction between the compressed stent and the surrounding sheath can be brought to a particularly low value.
Nevertheless, it is important for management of fatigue resistance to avoid imposing on any point of the stent tube a level of stress that is higher than the designed maximum. A stent tube made of metal is susceptible to fatigue failure, if only because it is subject to cyclic stress at the frequency of the heartbeat of the body in which it is installed. For this reason, regulatory authorities require stringent fatigue performance standards which impose on manufacturers of stents and delivery systems an onerous burden to avoid any unforeseen stresses on the stent tube.
The state of the art contains numerous suggestions to use an element within the lumen of the stent to restrain the stent from proximal withdrawal when the surrounding sleeve is withdrawn proximally. However, these systems are of interest only for bare stents, because they rely upon mechanical interaction between surfaces on the stent pusher within the stent lumen, and boundary surfaces of apertures within the wall thickness of the stent tube.
The present disclosure explains how to load self-expanding covered stents into catheter delivery systems which offers better management of stress within the stent tube, facilitates quality control and maintenance of sterile conditions, and is applicable to a range of stent tube designs. In a nutshell, a pusher is provided within the lumen of the covered stent, which exhibits protrusions along the length of the lumen, that interact with the covering of the stent along the length of the stent.
By distributing over the full length of the stent tube lumen the forces which necessarily have to be imposed on the stent in order to:    1. load it into a delivery sheath; and/or    2. restrain it from proximal movement during proximal withdrawal of the delivery sheath during placement of the stent at the stenting siteone can manage the distribution of stress within the stent tube so that it is distributed more or less homogeneously, rather than concentrated at one end of the stent tube. By using the covering of the stent as a link in the chain of stress distribution from the pusher to the sheath, one can further avoid any point at all within the metal stent tube which is subject to stress at a level higher than a prescribed design maximum. By their nature, stent coverings are more flexible than the stent tube itself, so have the capability to distribute stress from a point on a metallic stent pusher to an area, or volume, of the material of the stent tube.
Furthermore, the flexibility of the stent covering is sufficient to accommodate the protrusions of the pusher, irrespective where they lie in relation to the apertures of the stent lumen. With the present concept, there is no need to align in any way the protrusions of the stent pusher with the apertures of the stent lumen. Thus, a further technical effect is valuable simplicity and speed of operation in loading a range of different covered stent products into their corresponding delivery systems.
Yet a further advantage is that the stent pusher needs no undercut or rebated surfaces to achieve its effect, and the pusher has an outside diameter which is smaller than the inside or luminal diameter of the stent tube. These factors give greater reassurance that, when the stent has been placed, and the pusher has to be withdrawn from the stent lumen, there will be no inadvertent or unintended snagging of surfaces of the pusher on surfaces of the covered stent, or indeed of any bodily tissue that might impinge on the surfaces of the stent pusher after it has been withdrawn proximally out of the stent lumen.
Of particular interest is a stent pusher with protrusions arranged helically. Such protrusions will achieve the desired pushing effect when the pusher is subject to axial stress. However, arranging the protrusions helically would allow the pusher to be withdrawn from the stent lumen, even while the stent is within the sheath of the delivery system, simply by “unscrewing” the shaft of the pusher until the helical protrusions emerge, by continued rotation of the pusher relative to the stent, out of the lumen of the stent. In this way, one can employ the stent pusher as part of a system for loading a covered stent into a sheath, but then remove the pusher, and pass the sheath stent assembly onwards for incorporation into a delivery system which will use an entirely different stent pusher.
To assist understanding of the present invention, to be described below, and to assist in its realisation, reference will now be made to FIGS. 3 to 5 of the accompanying drawings (which come from above-mentioned PCT 486) and in which:
FIG. 3 is a side view of a tool for loading a covered self-expanding stent into a sheath;
FIG. 4 is an enlarged view of the distal end (II) of the tool of FIG. 3; and
FIG. 5 is an axial diametral section through the distal tip of a stent delivery system.
FIG. 5 shows only the distal tip of the delivery system, but the remainder of the system is not part of the contribution which the present invention makes to the art and, in any event, is familiar to those skilled in this art. The basic components of a conventional delivery system for a self-expanding stent are an inner catheter and an outer sheath, the purpose of the outer sheath being to confine the self-expanding stent radially, to the small radius delivery configuration, until its release at the site of stenting. The purpose of the inner catheter is to restrain the stent from proximal movement with the sheath, while the sheath is being withdrawn proximally.
Looking at FIG. 5 of the drawings, the outer sheath 10 of the delivery system has an integral tapered tip 12 which narrows down to an end ring 14 of a diameter appropriate to receive a guidewire (not shown). Confined within the sheath is a covered stent of which the structural foundation is a stent body 20 which is an apertured tube of nickel-titanium shape memory alloy. The stent is covered by an outer layer 22 of ePTFE on the abluminal surface of the stent body, and a covering layer 24 of ePTFE on the luminal inner surface of the stent body 20, with the inner and outer layers 24 and 22 being fused together where they can be pressed together within the apertures 26 of the stent body.
Between the luminal and abluminal surfaces of the stent body 20 is a wall thickness of the metallic stent material annulus. This annulus lies between the luminal and abluminal major surfaces of the stent body and, in the specification, we use the terminology “envelope” to indicate the generalised surfaces of the luminal and abluminal major wall surfaces of the stent body. Thus, the outer layer 22 lies outside the abluminal envelope stent body 20, except where it protrudes into the apertures 26 for fusing with the inner layer and, likewise, the inner layer 24 lies radially within the luminal envelope of the stent body 20 except where it protrudes radially outwardly into the stent body apertures 26.
The stent body carries a ring of tantalum radiopaque markers 28 at its distal end and a second ring of radiopaque tantalum markers 30 at its proximal end. It will be appreciated that the presence of these markers may further militate against pushing structures that bear against the end surface of the stent to be deployed.
The inner catheter 40 defines a guidewire lumen 42. Conveniently, the inner catheter 40 is based on a stainless steel hypo tube. This of course endows the entire delivery system with substantial pushability, but the hypo tube can also be made remarkably flexible for the desired trackability of the system through particularly tortuous bodily lumens. In any event, if stainless steel is not flexible enough for the distal zone of the delivery system, then it would be feasible to build the inner catheter 40 from other more flexible materials such as particular polymers.
The stent delivery system can be arranged as an over-the-wire system with a full length guidewire lumen, or a rapid exchange system with a guidewire lumen only in a distal zone of the system. The outer sheath 10 can be withdrawn by a full length outer catheter or a pull wire within a shaft lumen. For an example of delivery systems of the present Applicant, see WO 03/003944 and WO 2004/062458.
The inner catheter has an abluminal surface 44 which carries on it a wire 46 arranged as a helix so as to provide a plurality of protrusions (at least when seen in section as in the drawing) on the abluminal surface 44. In the illustrated embodiment, the wire is of stainless steel, fixed to the stainless steel tube 40 by deposits 50 of a bonding material which could be a weld bead or a suitable adhesive.
In any event, as can be seen on the drawing, when the stent body is radially inwardly compressed down onto the inner catheter 40, the inner ePTFE layer 24 deforms to accommodate the protrusions 48, but the protrusions 48 do not reach radially outwardly as far as the luminal envelope of the stent body 20.
In use, when the illustrated distal tip zone has been brought to the site of stenting, the outer catheter 12 is carefully and progressively withdrawn proximally so that the tip stretches and slides over the outer ePTFE layer 22 of the stent, progressively releasing the stent, starting at its distal end near the markers 28.
As the stent progressively expands, the inner ePTFE layer 24 moves radially outwardly away from the protrusions 48 until, with complete withdrawal of the tip 12 proximally beyond the proximal ring of radiopaque markers 30, the stent is fully released. It will be appreciated that there is then a substantial annular gap between the lumen of the expanded stent and the envelope containing the protrusions 48, enabling the inner catheter 40 also to be withdrawn proximally from the lumen of the stent without any snagging of the inner catheter 40 on any part of the stent.
It will be appreciated that, for loading a stent into a sheath, an analogous sequence of steps may be performed, with radially inward compression of the stent body down onto the protrusions 48 of a loading tool which has a shape in section analogous to that of the inner catheter 40. Once the stent has been so compressed, a suitable sheath can be offered up to one end of the compressed stent tube, and then the stent can be urged axially into the sheath by imposing an axial force on the line of protrusions 48 through the tube 40 on which they amounted, so that this force is transferred from the protrusions 48 to the inner layer 24 and thence to the stent body 20 and the outer layer 22, so that the entire covered stent device is urged by the protrusions 48 into the receiving sheath.
A particular advantage of the helical structure of protrusions 48 as shown in the drawing is that the pusher within the stent lumen can be removed trouble-free from the lumen of the stent even when it is in a compressed configuration within a sheath as shown in the drawing, simply by “unscrewing” the pusher from within the stent lumen.
Drawing FIGS. 3 and 4 show a suitable loading tool 60, long enough to push the covered stent along the full length of the outer catheter 10, after being compressed and introduced and advanced into the proximal end of the outer catheter. The tool 60 features at its distal end a radially-outwardly protruding wire spiral 62 with a configuration corresponding to that of the protrusions 48 and the inner catheter 40 (although non-corresponding configurations are also feasible). The covered stent is compressed around the protrusions 62 before the tool 60 is used to urge the covered stent by means of the protrusions 62, from the proximal to the distal end of the outer catheter.
The illustrated embodiment shows a system in which the tapered distal tip of the stent delivery system is carried on the distal end of the outer catheter. Those skilled in the art are well-aware that many proposed delivery systems feature a tapered tip on the inner catheter instead. The present invention is just as useful in such systems as it is in systems, as illustrated, with the tapered tip on the outer catheter.
The stent on which the present device operates can be an covered self-expanding stent. The stent which is the basis of the illustrated embodiment is the one that is the preferred embodiment of WO 2002/015820 which is cut from a nickel-titanium tube. However, the invention is equally applicable to other stent design philosophies, such as stents fabricated from wire (one example is the Gianturco “Z” stent made from zig zag wire rings) or other metals, such as stainless steel. The invention is particular useful for covered stents in which only the cover connects adjacent ones of a plurality of stenting rings, because the engagement of the pusher over the full length of the stent should avoid any tendency for the stent covering to “concertina” between the stenting rings when pushed only from its trailing (usually proximal) end.